1. Field of the Invention
The present invention relates to an image display device, and more specifically an image display device that has both of a function of area control for controlling a degree of light emitting luminance of a backlight according to luminance information in an image signal and a function of overdrive for reducing a blur of moving image, and displays an image using a liquid crystal display element.
2. Description of the Related Art
It has been proposed that an image display device displays an image using a liquid crystal display element at a frame frequency higher than a frame frequency of an input image signal to be converted into the high frame frequency, in order to improve moving image characteristics. In addition to the frame frequency conversion method, if an image signal is multiplied by an emphasis coefficient which is determined by a level difference between image signals of pixels in two adjacent frames on time axis, the moving image characteristics are further improved. The method for multiplying an image signal by the emphasis coefficient is generally called “overdrive”.
However, when the image display device combines use of the frame frequency conversion method and the overdrive, the overdrive processing overloads the image display device as the frame frequency becomes higher. Further, the frame frequency becomes higher, which increases a speed of accessing a memory where the image display device writes or reads frame images to be used for comparing at least two adjacent frames in the overdrive processing.
In order to resolve the above-described problems, there has been known an image display device in which a speed of accessing a memory or processing a signal is the same as one in a device in which a frame frequency is not converted into a higher frame frequency, by building an overdrive unit into an image signal processing unit into which an image signal is input before a frame frequency of the input image signal is converted into a high frame frequency. For example, this image display device is disclosed in Patent document 1 (Japanese Published Unexamined Application NO. 2006-337448). According to the conventional image display device, a memory, which is prepared for carrying out the overdrive processing after a frame frequency of the input image signal is converted into a high frame frequency, can be eliminated from the image display device by using frame data for detecting motion information and generating increment frames in the overdrive unit to carry out the overdrive processing.
In addition to the improvement in moving image characteristics, as a method for improving power consumption and contrast in an image display device provided with a backlight such as a liquid crystal display device, there has been known an image display device that divides each frame of a still image or moving image into a certain area, individually controls a backlight and an image signal level based on characteristics by area, and displays the still image or moving image thereon. For example, this image display device is disclosed in Patent document 2 (Japanese Published Unexamined Application No. 2007-133051). The improvement method in the conventional image display device provided with the backlight is called “area control”.
However, in the image display device disclosed in Patent document 1, since a frame frequency conversion unit (time series conversion memory) is arranged on the stage subsequent to the overdrive unit (time axis emphasis circuit), if circuits for greatly changing an image signal level, such as an image signal processing unit for adjusting a gain of image signal and a backlight luminance control unit for adjusting an amount of light to be emitted from a backlight device which are disclosed in Patent document 2, are arranged on the stage subsequent to the frame frequency conversion unit, the emphasis by overdrive to be suitable for the level difference between image signals of original image is weakened by area control. This causes the emphasis by overdrive to be excessive or deficient, which affects a moving image response and/or a moving image quality. Hereinafter, a gain adjustment of an image signal in the image signal processing unit and a light amount adjustment of the backlight device in the backlight luminance control unit are called “area control”.
For the overdrive processing, a response speed of a liquid crystal display element differs according to the degree of gradation change in a liquid crystal display device, which brings a difference of emphasis coefficient. Therefore, if an image signal which has been multiplied by an optimal emphasis coefficient in the overdrive unit is processed in an area control unit, there is a possibility that an emphasis by overdrive will be excessive or deficient. As a result, when an image of a captured object such as a car that moves at high speed is displayed, a fake contour occurs around a real contour of the captured object on the displayed image by excessive emphasis, or a blur of the captured object occurs by deficient emphasis.